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21 May 2017

Bob Blog 21 May

WEATHERGRAM
YOTREPS
Compiled Sun 21 May 2017

Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world.
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The state of the ENSO
In the Atmosphere:
The Southern Oscillation Index SOI (30 day running mean) sums up the weather pattern over the South Pacific as one number. It is based on the standardized difference in the barometer readings between Tahiti and Darwin. When the SOI is more than plus one (standard deviation from its mean) for more than a month we call it a LA NINA event, and when it stays more than minus one we call it an EL NINO event.

El Nino and La Nina are tropical influences on the weather: the La Nina shifts the subtropical ridge away from the equator and the El Nino draws the subtropical ridge closer to the equator. This affects the seasonal weather all around the planet.

From May 2015 to May 2016 we had an extreme El Nino. Then there was a weak La Nina in October 2016, and then we had weak plus/minus events. However over the past few weeks the SOI has dived to below minus 1 (-10 in the graph scale used here). If this value is held for three more weeks then we are having a new EL NINO.
A trend toward El Nino as seen at www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=soi&p=weekly

In the Ocean:
The eastern equatorial Pacific Ocean acts as a heat storage area during an El Nino, or becomes cooler than normal during a La Nina. This plays with the heat budget of the atmosphere and thus with the weather patterns.
There were warm seas during the El Nino of 2015/2016 and cooler seas late last year and earlier this year. Since Feb/Mar the seas in the target area have warmed, but not yet enough to call this an El NINO (as far as the ocean is concerned).
Seas are warm but not yet an El Nino, as seen at www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=nino34&p=monthly

The International Research Institute of the Climate Prediction Centre compile data from several ENSO prediction models. Looking at the Nino3-4 parameter, most models do NOT go for an El NINO, but they do go for higher than normal sea temperatures, with a reasonably flat trend for the next year.
CPC/IRI predictions are at
iri.columbia.edu/our-expertise/climate/forecasts/#ENSO_Forecasts

In summary: the atmosphere is starting to show some El Nino traits, but is only weakly being supported by the ocean. This means that although we may see some weather features typical of El Nino over the next month or so, there is also room for variation and other patterns to appear.

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The Monsoon
AT this time of year, the noon-day sun is over 20N-the thickest part of northern India. Up until now the cooler oceanic surface winds have been held off India mainland in spite of the extra heating from the overhead sun as it travelled north. One factor in this has been the jetstreams over 30N, the Himalayas. However, we have now entered that critical part of the year, where the jetstreams shift over the Himalayas.

Himalaya is the only part of planet earth that touches the stratosphere, and some call it our "third pole". Without jetstreams, Everest becomes climbable, and we are now entering the Everest climbing season (last around 7-10 days). This is such a critical event in the annual cycle of the planet (more so than a solstice or equinox) that some meteorologists like to use it as to demark the beginning of a new meteorological year. It seems that other seasonal parameters such as ENSO seem to start and end around this time of the year. Anyway, the absence of jetstreams over Himalaya is the very event that allows the cooler oceanic surface winds to finally enter India, bringing monsoonal rain. A Monsoon arrival map may be found at www.imd.gov.in/pages/monsoon_main.php and is updated almost daily. Enjoy.
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The tropics
It is amazing what difference one week can make, not just in politics, but also in tropical weather. Whereas last week we had cyclones in both hemispheres, this week things have quietened and there are no cyclones around (just a tropical low north of the Philippines).
There has been a build up of convection in the equatorial Indian Ocean and this is related to an MJO that is heading east (not monsoonal). At this time of year it should induce extra convection around the Philippines in a few weeks.
Rain for the past fortnight is available at trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif===============

WEATHER ZONES
SPCZ=South Pacific Convergence zone.
AN upper trough is building cloud over Fiji tonight. This is expected is to descend into a LOW that forms over Southern Tonga on Monday and the Niue area on Tuesday/Wednesday UTC, and then moves off to the SE. and fades. Avoid. Likely to be squally near this low.

Subtropical ridge (STR)
The force is strong with the STR this week. One High over northern New Zealand on Monday is expected to travel to east of NZ along 35S. There is likely to be a squash zone of enhanced SE winds on the northern side of this High from Monday to Thursday UTC.
Next HIGH from the Australian east coast on Mon 22 May is expected to travel east over central NZ on Wednesday 24 May.
Then another HIGH is expected to travel east along 25 to 30S across northern Tasman Sea from Fri 26 to Tue 30 may.

Australia to New Caledonia:
The GO EAST RALLY is likely to depart South Port on early Tuesday:
This should work thanks to a southerly component in the wind along the way on Wednesday as a trough forms between New Caledonia and Fiji.

Departing NZ to the north for the tropics:
Good opportunity for departing NZ is expected AFTER a trough passes off to the east on Thu 25 May.

New Zealand to the east (Tahiti)
Light winds are Ok for departure on Mon22/Tue 23 May. After that things get complicated.

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